A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern may be imaged onto a target portion (e.g. including part of, one or several dies) of a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that may be successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, and other devices. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in a track tool that typically applies a layer of resist to a substrate and develops the exposed resist, a metrology tool or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example, in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning device” used herein should be broadly interpreted as referring to any device that may be used to impart a projection beam with a pattern in its cross-section, such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
Patterning devices may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
The support structure supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as, for example, whether or not the patterning device is held in a vacuum environment. The support may include mechanical clamping, vacuum, or other clamping techniques, for example, electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable and which may ensure that the patterning device is at a desired position, for example, with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device”.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system”.
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
In order to image the pattern via the lens to the substrate, the layer of resist provided on the substrate should be in the focal plane of the projection system. Focus tests have been developed to test if the substrate is positioned correctly. A test pattern may be provided by a test device to be imaged on the layer of resist. Next, a latent image of the test pattern is made visible by performing, for instance, a post exposure bake. After this, the width of, for instance, a line of the created pattern may be measured using, for instance, a scanning electron microscope. By comparing this width with a previously obtained calibration graph (bossung curve) the defocus may be determined. It will be understood that the width of a line is minimal in the best focus position and will become larger with increasing defocus.
In some places, this text refers to positioning the substrate in the focal plane of the projection system. It is understood that this should be read as positioning the layer of resist provided on the substrate in the focal plane of the projection system.
However, in telecentric focus tests, it is only possible to determine the absolute defocus, but not the relative defocus. The absolute defocus is the distance between the layer of resist and the focal plane, but does not provide information on whether the resist is above or below the focal plane. In other words, the sign of the defocus can not be determined. The relative defocus is the distance between the layer of resist and the focal plane including the sign of the defocus. The relative defocus also provides information about whether the resist is above or below the focal plane. Telecentric focus tests only provide information about the absolute defocus and do not provide sufficient information for correcting the position of a substrate with respect to the projection system, since the sign of the absolute defocus is not known. Other problems exist.
US2002/0015158 A1 discloses a method of detecting focus information based on illumination rays having different main ray incidence directions, which may include the projection beam is tilted. Images of marks are projected through an optical system. A blocking member is provided in the illumination system that can be positioned in the light beam. The blocking member is provided with an aperture to partially block the light beam in such a way that a tilted beam is generated.
In order to determine the lateral shift of the projected mark images with respect to a mark image projected using a non tilted beam, a reference is needed. According to US2002/0015158 A1, this is achieved by superimposing a first and a second image of a single mark on the reticle. The mark is first projected on a substrate using a tilted beam, and next, another projection on the substrate using a non tilted beam is superimposed on the previous one. In between the first and second projections, the blocking member is removed from the light path. The lateral shift is given by the mutual distance between the first and second projected marks on the substrate.
According to a further embodiment of US2002/0015158 A1, a first exposure is carried out using a first blocking member, and a second exposure is carried out using a second blocking member, where the first and second blocking members have apertures that are opposite each other, such that the tilts of the first and second exposure are opposite each other. Thus the sensitivity of the measurement method is doubled.
The tilted focus test as disclosed in US2002/0015158 A1 has some disadvantages. First, the blocking member must specially be designed to take into account the relative position of the marks on the reticle. Thus, the blocking member needs to be designed with dependence on the relative position of the mark on the reticle.
Second, according to US2002/0015158 A1, double exposure is needed to provide a reference, in between which the blocking member needs to be removed or replaced with an other (opposite) blocking member. This is a time-consuming procedure that reduces the throughput of the system.
Third, determining the relative shift of projected marks on top of each other is a rather difficult procedure. Two relatively shifted lines that are projected on top of each other, may result in one single broader line. It is difficult to distinguish the change of line width due to the actual focus conditions and other effects, such as processing, dose variations, etc.
US2002/0100012 A1 describes several ways to create a tilted beam for use in a tilted focus test. A tilted beam is obtained by blocking certain diffraction orders. The blocking can, for instance, be achieved by positioning a pellicle under the mask. A frame member holding the pellicle is then used to block a diffraction order of the mark. According to an alternative, part of the normally transparent part of the pellicle can be non-transparent, to block certain diffraction orders.
The options presented in US2002/0100012 A1 have several disadvantages. Using the frame member to create a tilted beam is rather cumbersome and can only be used if the angle under which the diffraction orders are emitted by the mark and the distance between the frame member and the mark are within certain limits with respect to each other. Using the normally translucent pellicle to block certain diffraction orders requires a specially adapted pellicle for each mark. The overall performance of the pellicle will decrease as a result of the non-transparent parts having a negative effect on the over-all performance of the system. Also, pellicles can not be used in applications using (extreme) ultraviolet radiation beams, as will be understood by a skilled person.